Bluetooth low energy is a low-energy, low-cost radio communication technology that can be used, e.g., to collect information from sensors in an area. One possible setup is to have one Bluetooth low energy central device collecting information from several peripheral devices. In such a case the central device acts as a master and is connected to several peripheral devices which, once in a connection with the central device, act as slaves. To support a large number of peripheral devices it is in many cases advantageous if the master can maintain several simultaneous connections, each connection being associated with a different peripheral device.
The Industrial, Scientific and Medical (ISM) radio bands are radio bands reserved internationally for the use of Radio Frequency (RF) energy for industrial, scientific and medical purposes.
In recent years the fastest-growing uses of these bands have been for short-range, low power communications systems. Bluetooth devices use frequencies allocated to ISM, although this low power emitter is not considered ISM.
Bluetooth low energy is defined to operate in the 2.4 GHz ISM band, and uses approximately 80 MHz of the spectrum. In Bluetooth low energy there are 40 channels of 1 MHz; the channels being separated by 2 MHz. Out of these 40 channels, three are advertising channels, link layer channels 37, 38, and 39, used, e.g., to initiate a connection between a master node and a slave device, and 37 are data channels, link layer channels 0-36, used for payload exchange between the master node and the slave device. FIG. 1 depicts the Bluetooth low energy channel map with the respective link layer channel numbers. The first physical channel, which corresponds to the link layer channel 37, resides at a frequency of 2402 MHz, whereas the last physical channel, i.e., link layer channel 39, resides at 2480 MHz. A Bluetooth Low Energy (BLE) physical channel has a bandwidth of 1 MHz and uses GFSK modulation.
A connection in Bluetooth low energy is made up of connection events, which recur with a periodicity that is defined during the connection setup. The time in between two consecutive connection events is referred to as the Connection Interval (CI). Different connections may be configured using dissimilar parameters, e.g., using different connection intervals. A connection event is initiated by a packet transmission from master node to slave device and may comprise an arbitrary number of packet transmissions from the master node to the slave device and from slave device to master node. Furthermore, at each new connection event the channel number used for the packet transmissions is updated based on a frequency hopping algorithm. Frequency hopping is to change centre frequency for the carrier frequency of the transmitted signal at each hop interval over a bandwidth larger than required for the individual narrowband transmission. The frequency hopping is needed for two reasons, for regulatory purposes it allows the Bluetooth system to not implement a “listen-before-talk” scheme by periodically switching transmission frequency, and for performance purposes it gives some gains through frequency diversity as well as protection towards narrowband interference. FIG. 2 illustrates a situation in which a single master node communicates with two slave devices and how the connection events of the different connections are distributed in time. In the example in FIG. 2, the connection interval of connection 2 (CI2) is double compared to the connection interval of connection 1 (CI1). Furthermore, as illustrated in FIG. 2, time interleaving is used to separate connection events of different connections in time. The time interleaving is a means for the master node to distribute the connection events, related to different connected slave devices, in time.
FIG. 2 illustrates a schematic picture of two connections, CI1 and CI2 with their respective connection events, associated with the same master node but with different slave devices.
The channel usage of a data connection is defined by a frequency hopping algorithm. If some channels are considered as poor, e.g., due to high interference, it is possible for the master to mark these channels as poor; inform the slave about this and thereafter these channels are avoided by the frequency hopping algorithm. Bluetooth low adaptive frequency hopping technique was introduced to avoid interference, e.g., from other radio technologies such as WiFi that co-exist with Bluetooth in the 2.4 GHz band. FIG. 3 illustrates an example of frequency hopping in which the two connections referred to above are configured to avoid the link layer channel 24-32, which roughly corresponds to the WiFi channel 11. Every event on the time axis in FIG. 3 corresponds to a connection event.
FIG. 3 illustrates an example of adaptive frequency hopping.
To target an extension in coverage, i.e. longer distance and range, a simple principle is to trade data rate with range. When the data rate is reduced, the length of each connection event increases, provided that a fixed number of bits should be transmitted during the connection event, and hence fewer connection events can be supported. In addition, with an extended communication range a single central device such as a master node can reach out to a larger number of peripheral devices. Accordingly, for a longer range targeting system it becomes even more important that a large number of simultaneous connections are supported by the master node.